The Grand National

Imagine this: you queue for a roller coaster, only to find that at the boarding platform there're two cars to choose from. You pick your car, and then are sent haring round a woodentrack, fearing for you safety and sanity, racing the other car, the one you didn't pick.

I've tried experimenting to see which car wins, but it's different every time. I guess the only way to really guarantee being on the winning side is, whilst you're in the queue, to bribe all the fat (or more amply proportioned, I guess) riders to get on the car with you. The winch will always wind at the same speed - but you'll gather much more momentum on the way round.